1. Field of the Invention
This invention relates to a method of forming a metal pattern, more particularly to a method employing microwave energy to produce microwave plasma for acting on a patterned metal layer, thereby forming nanoparticles exhibiting a specific color in the patterned metal layer.
2. Description of the Related Art
Among existing printing methods, a printing method is chosen according to a material to be printed and a desired color. For example, when a paper sheet is to be printed, a printing medium, such as ink or laser carbon powder, is printed on the paper sheet, which can be conducted by using spray printing technique or screen-printing technique. However, when the material to be printed is not a paper sheet but other materials, an oil-based printing material is used so as to tightly attach thereto. In addition, when the desired color is not a single primary color, various colors are to be created by mixing different colors indifferent ratios. Therefore, there is a need to find a method of making a desired color pattern on different materials that are to be printed.
Nano-material usually includes nanoparticles, nanofiber, nano-film, and nano-bulk. Among others, since nanoparticles have been developed for a longer period of time, technologies thereof are more mature than others. Further, as nanofiber and nano-film are made from nanoparticles, production of nanoparticles is relatively important. In general, methods of producing nanoparticles are classified into physical method and chemical method.
A major example of chemical method is chemical reduction. In the chemical reduction, nanoparticles are formed through reduction of metal ions in a solution, to which a protecting agent is added so as to maintain uniform distribution of the nanoparticles therein and prevent aggregation of the nanoparticles. After the nanoparticles are covered by the protecting agent, a substrate, which has a surface modified with an organic functional group, is provided for formation of a self-assembly nanostructure, such as nanoparticles, thereon through static attraction force and chemical bonding therebetween. Solutions containing organic materials, such as toluene and thiol-containing organic molecules, are usually used in the chemical reduction. However, the organic materials are likely to contaminate the environment and are harmful to human health.
Examples of physical methods for producing nanoparticles include high temperature annealing, electron beam irradiation, heavy ion irradiation, pulsed laser irradiation, and nanolithography. In the first four of the physical methods, a thin film is heated so as to form cracks, become discontinuous, and be melted. Thereafter, spherical nanoparticles are formed by surface tension forces. In the last one of the physical methods, a substrate is covered by a specific mask. For example, nano-scale silicon particles are arranged in a hexagonal closed-packed structure. Subsequently, a metal is deposited on interstices of the hexagonal closed-packed structure such that the nanoparticles are formed and arranged in a triangular array. However, the above-mentioned five physical methods have the following disadvantages.
In the high temperature annealing method, raising and lowering temperature require a long period of time, which results in time-consumption and low efficiency, and non-uniform morphology and easy aggregation of the nanoparticles.
In the electron beam irradiation method, expensive equipment, such as an electron gun, is needed. In addition, since an electron beam generated from the electron gun can merely focus on a limited region on the substrate in each operation, a long time is required for producing nanoparticles on the substrate having a large area. Thus, the method is also less effective.
In the heavy ion irradiation method, the disadvantages are similar to those in the electron beam irradiation method, and the application thereof is still limited to academic study.
The pulsed laser irradiation method is also less effective because a laser source can irradiate only a small region of the substrate and needs to move forth and back to treat a large area of the substrate.
In the nanolithography method, although mass production of nanoparticles is possible, the method is complicated and time-consuming, and requires organic solvents to clean the substrate, which is not environmentally friendly.